1. Field of the Invention
The present invention relates generally to a method for absorbing the moisture of a composite material so that the composite material is in a moisture absorption state, which is equal to that of a composite material after the elapse of predetermined years under actual environment, when a test for evaluating the strength of the composite material is conducted.
2. Description of Related Background Art
With respect to the composite material structure of airplanes, it is required to insure a predetermined strength after the elapse of a predetermined useful life, so that it is required to reproduce a composite material, which is equal to a composite material after the elapse of predetermined years, e.g., 25 years, under actual environment, to conduct a test for evaluating the strength of the reproduced composite material. The moisture absorption state of a composite material after the elapse of predetermined years under actual environment is greatly different from that of a new composite material. This moisture absorption state of the composite material has a great influence on the strength of the composite material. Therefore, in order to conduct a test for evaluating the strength of a composite material, it is required to conduct the test in a moisture absorption state which is equal to that of a composite material which is placed under actual environment.
In order to obtain a composite material of such a high moisture absorption state, a composite material is conventionally placed under a high humidity environment, which includes a temperature of 71xc2x0 C. and a relative humidity of 95%RH or higher under atmospheric pressure, for several weeks so that the total quantity of moisture absorption is equal to that of a composite material under actual environment.
The composite material structure has a thick-wall portion having a thickness of 10 mm or more. usually, this thick-wall portion serves as a strength evaluated portion. In recent years, the thickness of the thick-wall portion of the composite material structure for use in airplanes increases, so that the moisture absorption period required to obtain a moisture absorption state, which is equal to that under actual environment, extends over a long period of time, e.g., there are some cases where a moisture absorption period of tens weeks to one year is required. Thus, there is a problem in that the development period extends over a long period of time. Therefore, it is desired to provide a method for absorbing the moisture of a composite material, which can absorb the moisture of the composite material in a short period of time.
In addition, there is a problem in that the moisture absorption distribution of a composite material placed under actual environment is different from that of a composite material which was moisture-absorbed in a short period of time.
FIG. 4 is a graph showing the moisture absorption distribution of a composite material in directions perpendicular thereto, wherein line 1 shows the moisture absorption distribution of a composite material after the elapse of 25 years under actual environment, and line 2 shows the moisture absorption distribution of a composite material which was moisture-absorbed for 36 weeks under a high humidity environment including a relative humidity of 95%RH. Furthermore, the axis of ordinates denotes the percentage of a coefficient of moisture absorption to the maximum coefficient of moisture absorption, which shows the moisture content of a composite material per unit weight. The axis of abscissas denotes the distance from the surface of the composite material, which denotes the distance from the surface to the center of the composite material, i.e., to 10 mm, since the thickness of the composite material is 20 mm.
As can be seen from this graph, the difference between the coefficients of moisture absorption of the surface and central portion of the composite material under actual environment is not so great, whereas the difference between the coefficients of moisture absorption of the surface and central portion of the composite material, which was moisture-absorbed in a short period of time under a high humidity environment, is greater. That is, the composite material, which was moisture-absorbed in a short period of time, can not accurately reproduce the moisture absorption state of the composite material under actual environment. Therefore, there is a problem in that the strength of the composite material, which is moisture-absorbed by the conventional moisture-absorbing method, can not be accurately evaluated as the thickness thereof increases. In particularly such a problem is remarkably caused in the case of a composite material having a thickness of 10 mm or more.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a method for absorbing the moisture of a composite material, which can absorb the moisture of the composite material in a short period of time and which can provide a moisture absorption distribution which is equal to that of a composite material which is placed under actual environment.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a method for absorbing the moisture of a composite material under a high humidity environment so that the moisture absorption state of the composite material is equal to the moisture absorption state of a composite material after the elapse of predetermined years under actual environment, wherein the moisture of the composite material is absorbed in a pressure container at a predetermined high temperature, a predetermined high humidity and a predetermined high pressure.
The predetermined high temperature may be a temperature below the glass transition point of the composite material.
The predetermined high temperature is preferably a temperature which is higher than 50xc2x0 C. and lower than 140xc2x0 C. when the composite material is made of an epoxy resin.
The predetermined high humidity may be a relative humidity of 70%RH or higher to 100%RH or lower.
The predetermined high pressure may be a pressure in the range of from 1400 hPa to 5000 hPa.
The predetermined high pressure is preferably a pressure in the range of from 1400 hPa to 3000 hPa.
According to another aspect of the present invention, there is provided a method for absorbing the moisture of a composite material under a high humidity environment so that the moisture absorption state of the composite material is equal to the moisture absorption state of a composite material after the elapse of predetermined years under actual environment, wherein the moisture of the composite material is absorbed in a pressure container at a predetermined high temperature of the glass transition point of the composite material or lower, a predetermined high humidity of a relative humidity in the range of from 70%RH to 100%RH, and a predetermined high pressure of higher than atmospheric pressure.
According to a further aspect of the present invention, there is provided a method for absorbing the moisture of a composite material under a high humidity environment so that the moisture absorption state of the composite material is equal to the moisture absorption state of a composite material after the elapse of predetermined years under actual environment, wherein the composite material in a pressure vessel, and the temperature, humidity and pressure in the pressure vessel are changed in accordance with a predetermined time table so that the moisture absorption distribution of the composite material in directions perpendicular thereto is equal to that of a composite material after the elapse of predetermined years under actual environment.
The time table may be prepared by a numerical operation using an equation indicative of Fick law, Arrhenius equation, an equation indicative of the relationship between a relative humidity and a saturated coefficient of moisture absorption, and an equation indicative of the relationship between a coefficient of moisture absorption and a moisture concentration.
According to the present invention, it is possible to provide a desired moisture absorption distribution by changing conditions for temperature, humidity and pressure in a pressure vessel. Thus, it is possible to obtain a composite material having a required moisture absorption distribution which is equal to that of a composite material after the elapse of predetermined years under actual environment, and it is possible to accurately evaluate the strength of a composite material even if the composite material has a great thickness.